The aim of the research program is to investigate in an animal model the chemical and neural control of the respiratory system during fetal life in utero; throughout birth, and in the immediate post natal period, and to examine the influence of respiratory activity and lung liquid flow on the maturation of the respiratory system. Such studies should provide an experimental background against which to examine the problems of apnea in the premature newborn; the sudden infant death syndrome; and the possibility that respiratory movements 'in utero' may be used as an indication of fetal well being. The experimental model is a chronic fetal sheep preparation in which lung liquid flow and pressure, electrical activity of the respiratory muscles, E.C.G. and vascular pressures and blood samples are measured at intervals over the last third of gestation, during birth and in the post-natal period. The activity of the respiratory system is monitored and it may be stimulated by chemical and mechanical stress in control preparations and those in which afferent, efferent and central neural pathways have been sectioned. Selective caesarian section of the preparation at intervals throughout gestation enables a parallel morphological study to be undertaken. Aberrant development and maturation of the respiratory system, the central nervous system and the cardiovascular system are recurring themes in the consideration of problems of the newborn. This program seeks to analyse the interaction of the first two of these sytems throughout development.